Preparation of cerium-containing precursor and deposition of cerium-containing films

ABSTRACT

Methods and compositions for depositing rare earth metal-containing layers are described herein. In general, the disclosed methods deposit the precursor compounds comprising rare earth-containing compounds using deposition methods such as chemical vapor deposition or atomic layer deposition. The disclosed precursor compounds include a cyclopentadienyl ligand having at least one aliphatic group as a substituent and an amidine ligand.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a divisional application of U.S. applicationSer. No. 14/039,692 filed Sep. 27, 2013, which is a continuationapplication of U.S. application Ser. No. 13/328,586 filed Dec. 16, 2011,which is a continuation-in-part application of U.S. application Ser. No.12/479,175 filed Jun. 5, 2009, which claims the benefit of U.S.Provisional Application Ser. No. 61/059,214 filed Jun. 5, 2008, each ofwhich is incorporated by reference herein in their entireties for allpurposes.

BACKGROUND

One of the serious challenges the industry faces is developing new gatedielectric materials for Dynamic Random Access Memory (DRAM) andcapacitors. For decades, silicon dioxide (SiO₂) was a reliabledielectric, but as transistors have continued to shrink and thetechnology moved from “Full Si” transistor to “Metal Gate/High-k”transistors, the reliability of the SiO₂-based gate dielectric isreaching its physical limits. The need for new high dielectric constantmaterial and processes is increasing and becoming more and more criticalas the size for current technology is shrinking. New generations ofoxides especially based on lanthanide-containing materials are thoughtto give significant advantages in capacitance compared to conventionaldielectric materials.

Nevertheless, deposition of lanthanide-containing layers is difficultand new material and processes are increasingly needed. For instance,atomic layer deposition (ALD) has been identified as an important thinfilm growth technique for microelectronics manufacturing, relying onsequential and saturating surface reactions of alternatively appliedprecursors, separated by inert gas purging. The surface-controllednature of ALD enables the growth of thin films having high conformalityand uniformity with an accurate thickness control. The need to developnew ALD processes for rare earth materials is obvious.

Unfortunately, the successful integration of compounds into depositionprocesses has proven to be difficult. Two classes of molecules aretypically proposed: beta-diketonates and cyclopentadienyls. The formerfamily of compounds is stable, but the melting points always exceed 90°C., making them impractical. Lanthanide2,2-6,6-tetramethylheptanedionate's [La(tmhd)₃] melting point is as highas 260° C., and the related lanthanide 2,2,7-trimethyloctanedionate's[La(tmod)₃] melting point is 197° C. Additionally, the deliveryefficiency of beta-diketonates is very difficult to control.Non-substituted cyclopentadienyl compounds also exhibit low volatilitywith a high melting point. Molecule design may both help improvevolatility and reduce the melting point. However, in process conditions,these classes of materials have been proven to have limited use. Forinstance, La(iPrCp)₃ does not allow an ALD regime above 225° C.

Some of the lanthanide-containing precursors currently available presentmany drawbacks when used in a deposition process. For instance,fluorinated lanthanide precursors can generate LnF₃ as a by-product.This by-product is known to be difficult to remove.

Consequently, there exists a need for alternate precursors fordeposition of lanthanide-containing films.

SUMMARY

Disclosed herein are lanthanide-containing precursors of the generalformula:

Ln(R¹Cp)_(m)(R²—N—C(R⁴)═N—R²)_(n),

wherein:

-   -   Ln=Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb,        Lu;    -   R¹ is selected from the group consisting of H and a C1-C5 alkyl        chain;    -   R² is selected from the group consisting of H and a C1-C5 alkyl        chain;    -   R⁴ is selected from the group consisting of H and Me;    -   n and m range from 1 to 2; and    -   the precursor has a melting point below approximately 105° C.    -   the precursor being Ce(iPrCp)₂(iPr-N—C(H)═N-iPr).    -   the precursor being Pr(iPrCp)₂(iPr-N—C(H)═N-iPr).

The disclosed lanthanide-containing precursors may optionally includeone or more of the following aspects:

-   -   Ln being Ce.    -   Ln being Pr.    -   Ln being selected from the group consisting of Lu, Gd, Tb, Dy,        Ho, Er, Tm, and Yb.    -   Ln being selected from the group consisting of Er and Yb.    -   R¹ being selected from the group consisting of Me, Et, and iPr.    -   R² being selected from the group consisting of iPr and tBu.

Also disclosed is a method for depositing a lanthanide-containing filmon a semiconductor substrate, the method comprising:

a) providing a substrate,

b) providing the disclosed lanthanide-containing precursor and

c) depositing a lanthanide-containing film on the substrate.

The disclosed method may optionally include one or more of the followingaspects:

-   -   depositing the lanthanide-containing film on the substrate at a        temperature between about 150° C. and about 600° C.    -   depositing the lanthanide-containing film on the substrate at a        pressure between about 0.5 mTorr and about 20 Torr.    -   the lanthanide-containing precursor being a liquid at a        temperature below 70° C.    -   the lanthanide-containing precursor being a liquid at a        temperature below 40° C.    -   the lanthanide-containing film being selected from the group        consisting of Ln₂O₃, (LnLn′)O₃, Ln₂O₃-Ln′₂O₃, LnSi_(x)O_(y),        LnGe_(x)O_(y), (Al, Ga, Mn)LnO₃, HfLnO_(x), and ZrLnO_(x),        wherein Ln and Ln′ are different.    -   the lanthanide-containing film being selected from the group        consisting of HfLnO_(x) and ZrLnO_(x).    -   the lanthanide-containing precursor having the general formula        selected from the group consisting of Ln(R¹Cp)₂(N^(Z)-fmd),        Ln(R¹Cp)₂(N^(Z)-amd), Ln(R¹Cp)(N^(Z)-fmd)₂, and        Ln(R¹Cp)(N^(Z)-amd)₂, wherein R¹ is selected from the group        consisting of Me, Et, and iPr; and Z is iPr or tBu.    -   the lanthanide-containing precursor being        Ce(iPrCp)₂(iPr-N—C(H)═N-iPr).    -   the lanthanide-containing precursor being        Pr(iPrCp)₂(iPr-N—C(H)═N-iPr).

Also disclosed is a second method of forming a lanthanide-containingfilm on a substrate comprising the steps of providing a reactor havingat least one substrate disposed therein, introducing at least onelanthanide-containing precursor disclosed herein into the reactor, andcontacting the lanthanide-containing precursor and the substrate to forma lanthanide-containing layer on at least one surface of the substrateusing a deposition process.

The disclosed second method may optionally include one or more of thefollowing aspects:

-   -   providing at least one oxygen containing fluid into the reactor        and reacting the lanthanide-containing precursor with the oxygen        containing fluid.    -   the oxygen containing fluid being selected from the group        consisting of O₂, O₃, H₂O, H₂O₂, acetic acid, formalin,        para-formaldehyde, and combinations thereof.    -   the lanthanide-containing precursor and the reactant species        being either introduced at least partially simultaneously as in        a chemical vapor deposition process, or are introduced at least        partially sequentially as in an atomic layer deposition process.    -   introducing a metal precursor into the reactor, wherein the        metal precursor is different than the lanthanide-containing        precursor, and depositing at least part of the metal precursor        to form the lanthanide-containing layer on the one or more        substrates.    -   a metal of the metal precursor being selected from the group        consisting of Hf, Si, Al, Ga, Mn, Ti, Ta, Bi, Zr, Pb, Nb, Mg,        Sr, Y, Ba, Ca, a lanthanide, and combinations thereof.    -   the deposition process being a chemical vapor deposition        process.    -   the deposition process being an atomic layer deposition process        having a plurality of deposition cycles.    -   the lanthanide-containing precursor having the general formula        selected from the group consisting of Ln(R¹Cp)₂(N^(Z)-fmd),        Ln(R¹Cp)₂(N^(Z)-amd), Ln(R¹Cp)(N^(Z)-fmd)₂, and        Ln(R¹Cp)(N^(Z)-amd)₂, wherein R¹ is selected from the group        consisting of Me, Et, and iPr; and Z is iPr or tBu.    -   the lanthanide-containing precursor being        Ce(iPrCp)₂(iPr-N—C(H)═N-iPr).    -   the lanthanide-containing precursor being        Pr(iPrCp)₂(iPr-N—C(H)═N-iPr).

Also disclosed are lanthanide-containing film coated substratescomprising the product of the disclosed second method.

Notation and Nomenclature

Certain abbreviations, symbols, and terms are used throughout thefollowing description and claims and include: the abbreviation “Ln”refers to the lanthanide group, which includes the following elements:scandium (“Sc”), yttrium (“Y”), lutetium (“Lu”), lanthanum (“La”),cerium (“Ce”), praseodymium (“Pr”), neodymium (“Nd”), samarium (“Sm”),europium (“Eu”), gadolinium (“Gd”), terbium (“Tb”), dysprosium (“Dy”),holmium (“Ho”), erbium (“Er”), thulium (“Tm”), or ytterbium (“Yb”); theabbreviation “Cp” refers to cyclopentadiene; the abbreviation “Å” refersto angstroms; prime (“′”) is used to indicate a different component thanthe first, for example (LnLn′)O₃ refers to a lanthanide oxide containingtwo different lanthanide elements; the term “aliphatic group” refers toa C1-C5 linear or branched chain alkyl group; the term “alkyl group”refers to saturated functional groups containing exclusively carbon andhydrogen atoms; the abbreviation “Me” refers to a methyl group; theabbreviation “Et” refers to an ethyl group; the abbreviation “Pr” refersto a propyl group; the abbreviation “iPr” refers to an isopropyl group;the abbreviation “tBu” refers to a tertiary butyl group, theabbreviation “N^(Z)-amd” refers to ZNC(CH₃)═NZ, wherein Z is a definedalkyl group such as iPr or tBu; the abbreviation “N^(Z)-fmd” refers toZNC(H)═NZ, wherein Z is a defined alkyl group such as iPr or tBu; theabbreviation “CVD” refers to chemical vapor deposition; the abbreviation“LPCVD” refers to low pressure chemical vapor deposition; theabbreviation “ALD” refers to atomic layer deposition; the abbreviation“P-CVD” refers to pulsed chemical vapor deposition; the abbreviation“PE-ALD” refers to plasma enhanced atomic layer deposition; theabbreviation “MIM” refers to Metal Insulator Metal (a structure used incapacitors); the abbreviation “DRAM” refers to dynamic random accessmemory; the abbreviation “FeRAM” refers to ferroelectric random accessmemory; the abbreviation “CMOS” refers to complementarymetal-oxide-semiconductor; the abbreviation “THF” refers totetrahydrofuran; the abbreviation “TGA” refers to thermogravimetricanalysis; the abbreviation “TMA” refers to trimethyl aluminum; theabbreviation “TBTDET” refers to tertiary butylimido, tris(diethylamino)tantalum (Ta[N(C₂H₅)₂]₃[NC(CH₃)₃]); the abbreviation “TAT-DMAE” refersto tantalum tetraethoxide dimethylaminoethoxide; the abbreviation “PET”refers to pentaethoxy tantalum; the abbreviation “TBTDEN” refers totertiary butylimido, tris(diethylamino) niobium; the abbreviation “PEN”refers to pentaethoxy niobium; the abbreviation “TriDMAS” refers totris(dimethylamino) silane [SiH(NMe₂)₃]; the abbreviation “BDMAS” refersto bis(dimethylamino) silane; the abbreviation “BDEAS” refers tobis(diethylamino) silane [SiH₂(NEt₂)₂]; the abbreviation “TDEAS” refersto tetrakis-diethylamino silane; the abbreviation “TDMAS” refers totris(dimethylamino) silane; the abbreviation “TEMAS” refers totetrakis-ethylmethylamino silane (Si(N(C₂H₅)(CH₃))₄); the abbreviation“BTBAS” refers to bis(tert-butylamino)silane [SiH₂(NHtBu)₂].

BRIEF DESCRIPTION OF THE DRAWINGS

For a further understanding of the nature and objects of the presentinvention, reference should be made to the following detaileddescription, taken in conjunction with the accompanying drawings,wherein:

FIG. 1 is a TGA graph demonstrating the percentage of weight loss withtemperature change of Y(MeCp)₂(N^(iPr)-amd).

FIG. 2 is a TGA graph for Y(iPrCp)₂(N^(iPr)-amd).

FIG. 3 is a TGA graph for Er(MeCp)₂(iPr-N—C(Me)=N-iPr).

FIG. 4 is a TGA graph for Er(MeCp)₂(tBu-N—C(Me)=N-tBu).

FIG. 5 is a TGA graph for Er(EtCp)₂(iPr-N—C(Me)=N-iPr).

FIG. 6 is a TGA graph for Er(MeCp)₂(iPr-N—C(H)═N-iPr).

FIG. 7 is a TGA graph for Yb(MeCp)₂(iPr-N—C(Me)=N-iPr).

FIG. 8 is a TGA graph for Yb(MeCp)₂(tBu—N—C(Me)=N-tBu).

FIG. 9 is a TGA graph for Yb(EtCp)₂(iPr-N—C(Me)=N-iPr).

FIG. 10 is a TGA graph for Yb(EtCp)₂(iPr-N—C(H)═N-iPr).

FIG. 11 is a TGA graph for Yb(iPrCp)₂(iPr-N—C(H)═N-iPr).

FIG. 12 is a TGA graph for Ce(iPrCp)₂(iPr-N—C(H)═N-iPr).

FIG. 13 is a TGA graph for Pr(iPrCp)2(iPr-N—C(H)+N-iPr).

DESCRIPTION OF PREFERRED EMBODIMENTS

Disclosed are lanthanide-containing precursor compounds having thegeneral formula:

Ln(R¹Cp)_(m)(R²—N—C(R⁴)═N—R²)_(n),

wherein Ln represents the lanthanide group, which includes Sc, Y, La,Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu; R¹ is selected fromH or a C1-C5 alkyl chain; R² is selected from H or a C1-C5 alkyl chain;R⁴ is selected from H or a C1-C5 alkyl chain; m is selected from 1 or 2;and n is selected from 1 or 2.

The lanthanide-containing precursors offer unique physical and chemicalproperties when compared to their corresponding homoleptic compounds,which include tris-substituted cyclopentadienyl lanthanide compounds,Ln(RCp)₃, tris-acetamidinate compounds, Ln(R—N—C(R′)═N—R)₃, ortris-formamidinate compounds, Ln(R—N—C(H)═N—R)₃. Such properties includebetter control of steric crowding around the metal center, which in turncontrols the surface reaction on the substrate and the reaction with asecond reactant (such as an oxygen source). Independently fine tuningthe substituents on the ligands increases volatility and thermalstability and decreases melting point to yield either liquids or lowmelting solids (having a melting point below approximately 105° C.).

In order to synthesize stable lanthanide-containing precursors withproperties suited for the vapor deposition process (i.e, a volatile, yetthermally stable, liquid or low melting solid (having a melting pointbelow about 105° C.)), a direct correlation between the properties ofthe central metal ion (coordination number) and ligands (steric effect,ratio of two heteroleptic ligands) has been observed. Preferably, themetal compound has a 3+ charge and coordination number of 6. As aresult, Ln is preferably Ce. Preferably R¹ is a C1-C3 alkyl chain; R² isa C3-C4 alkyl chain, and R⁴ is H or Me. Preferably thelanthanide-containing precursor has a melting point below about 105° C.,preferably below about 80° C., more preferably below about 70° C., andeven more preferably below about 40° C. Preferred lanthanide-containingprecursors include Ln(R¹Cp)₂(N^(Z)-fmd), Ln(R¹Cp)₂(N^(Z)-amd),Ln(R¹Cp)(N^(Z)-fmd)₂, and Ln(R¹Cp)(N^(Z)-amd)₂, wherein R¹ is Me, Et, oriPr; and Z is iPr or tBu.

The synthesis of the Ln(R¹Cp)_(m)(R²—N—C(R⁴)═N—R²)_(n) precursor (wherem=2, n=1 or m=1, n=2) may be carried out by following methods:

Method A

By reacting Ln(R¹Cp)₂X (where X═Cl, Br or I) with M(R²—N—C(R⁴)═N—R²)(where M=Li, Na, K) or by reacting Ln(R¹Cp)X₂ with 2M(R²—N—C(R⁴)═N—R²)(Scheme-1).

Method B

By reacting Ln(R¹Cp)₃ with one equivalent of amidine/guanidine,R²—NH—C(R⁴)═N—R², to yield Ln(R¹Cp)₂(R²—N—C(R⁴)═N—R²) or with twoequivalents of amidine/guanidine, R²—NH—C(R⁴)═N—R², to yieldLn(R¹Cp)(R²—N—C(R⁴)═N—R²)₂ (Scheme-2).

Method C

In-situ reacting LnX₃ (where X═Cl, Br, I) (in a stepwise reactionwithout isolation of intermediate products) with mR¹CpM (where M=Li, Na,K) followed by filtration, and reacting the filtrate withnM(R²—N—C(R⁴)═N—R²) to result in Ln(R¹Cp)_(m)(R²—N—C(R⁴)═N—R²)_(n)precursor (Scheme-3).

The disclosed precursor compounds (hereinafter the“lanthanide-containing precursor”) may be deposited to formlanthanide-containing films using any deposition methods known to thoseof skill in the art. Examples of suitable deposition methods includewithout limitation, conventional chemical vapor deposition (CVD), lowpressure chemical vapor deposition (LPCVD), atomic layer deposition(ALD), pulsed chemical vapor deposition (P-CVD), plasma enhanced atomiclayer deposition (PE-ALD), or combinations thereof.

The type of substrate upon which the lanthanide-containing film will bedeposited will vary depending on the final use intended. In someembodiments, the substrate may be chosen from oxides which are used asdielectric materials in MIM, DRAM, FeRam technologies or gatedielectrics in CMOS technologies (for example, HfO₂ based materials,TiO₂ based materials, ZrO₂ based materials, rare earth oxide basedmaterials, ternary oxide based materials, etc.) or from nitride-basedfilms (for example, TaN) that are used as an oxygen barrier betweencopper and the low-k layer. Other substrates may be used in themanufacture of semiconductors, photovoltaics, LCD-TFT, or flat paneldevices. Examples of such substrates include, but are not limited to,solid substrates such as metal substrates (for example, Au, Pd, Rh, Ru,W, Al, Ni, Ti, Co, Pt and metal silicides, such as TiSi₂, CoSi₂, andNiSi₂); metal nitride containing substrates (for example, TaN, TiN, WN,TaCN, TiCN, TaSiN, and TiSiN); semiconductor materials (for example, Si,SiGe, GaAs, InP, diamond, GaN, and SiC); insulators (for example, SiO₂,Si₃N₄, SiON, HfO₂, Ta₂O₅, ZrO₂, TiO₂, Al₂O₃, and barium strontiumtitanate); or other substrates that include any number of combinationsof these materials. The actual substrate utilized may also depend uponthe specific precursor embodiment utilized. In many instances though,the preferred substrate utilized will be selected from TiN, Ru, and Sitype substrates.

The lanthanide-containing precursor is introduced into a reactionchamber containing at least one substrate. The reaction chamber may beany enclosure or chamber of a device in which deposition methods takeplace, such as, without limitation, a parallel-plate type reactor, acold-wall type reactor, a hot-wall type reactor, a single-wafer reactor,a multi-wafer reactor, or other such types of deposition systems.

The reaction chamber may be maintained at a pressure ranging from about0.5 mTorr to about 20 Torr. In addition, the temperature within thereaction chamber may range from about 250° C. to about 600° C. One ofordinary skill in the art will recognize that the temperature may beoptimized through mere experimentation to achieve the desired result.

The substrate may be heated to a sufficient temperature to obtain thedesired lanthanide-containing film at a sufficient growth rate and withdesired physical state and composition. A non-limiting exemplarytemperature range to which the substrate may be heated includes from150° C. to 600° C. Preferably, the temperature of the substrate remainsless than or equal to 450° C.

The lanthanide-containing precursor may be fed in liquid state to avaporizer where it is vaporized before it is introduced into thereaction chamber. Prior to its vaporization, the lanthanide-containingprecursor may optionally be mixed with one or more solvents, one or moremetal sources, and a mixture of one or more solvents and one or moremetal sources. The solvents may be selected from the group consisting oftoluene, ethyl benzene, xylene, mesitylene, decane, dodecane, octane,hexane, pentane, or others. The resulting concentration may range fromapproximately 0.05 M to approximately 2 M. The metal source may includeany metal precursors now known or later developed.

Alternatively, the lanthanide-containing precursor may be vaporized bypassing a carrier gas into a container containing thelanthanide-containing precursor or by bubbling the carrier gas into thelanthanide-containing precursor. The carrier gas andlanthanide-containing precursor are then introduced into the reactionchamber. If necessary, the container may be heated to a temperature thatpermits the lanthanide-containing precursor to be in its liquid phaseand to have a sufficient vapor pressure. The carrier gas may include,but is not limited to, Ar, He, N₂, and mixtures thereof. Thelanthanide-containing precursor may optionally be mixed in the containerwith a solvent, another metal precursor, or a mixture thereof. Thecontainer may be maintained at temperatures in the range of, forexample, 0-100° C. Those skilled in the art recognize that thetemperature of the container may be adjusted in a known manner tocontrol the amount of lanthanide-containing precursor vaporized.

In addition to the optional mixing of the lanthanide-containingprecursor with solvents, metal precursors, and stabilizers prior tointroduction into the reaction chamber, the lanthanide-containingprecursor may be mixed with reactant species inside the reactionchamber. Exemplary reactant species include, without limitation, H₂,metal precursors such as TMA or other aluminum-containing precursors,other lanthanide-containing precursors, TBTDET, TAT-DMAE, PET, TBTDEN,PEN, and any combination thereof.

When the desired lanthanide-containing film also contains oxygen, suchas, for example and without limitation, erbium oxide, the reactantspecies may include an oxygen source which is selected from, but notlimited to, O₂, O₃, H₂O, H₂O₂, acetic acid, formalin, para-formaldehyde,and combinations thereof.

When the desired lanthanide-containing film also contains nitrogen, suchas, for example and without limitation, erbium nitride or erbiumcarbo-nitride, the reactant species may include a nitrogen source whichis selected from, but not limited to, nitrogen (N₂), ammonia and alkylderivatives thereof, hydrazine and alkyl derivatives thereof,N-containing radicals (for instance N., NH., NH₂.), NO, N₂O, NO₂,amines, and any combination thereof.

When the desired lanthanide-containing film also contains carbon, suchas, for example and without limitation, erbium carbide or erbiumcarbo-nitride, the reactant species may include a carbon source which isselected from, but not limited to, methane, ethane, propane, butane,ethylene, propylene, t-butylene, isobutylene, CCl₄, and any combinationthereof.

When the desired lanthanide-containing film also contains silicon, suchas, for example and without limitation, erbium silicide, erbiumsilico-nitride, erbium silicate, erbium silico-carbo-nitride, thereactant species may include a silicon source which is selected from,but not limited to, SiH₄, Si₂H₆, Si₃H₈, TriDMAS, BDMAS, BDEAS, TDEAS,TDMAS, TEMAS, (SiH₃)₃N, (SiH₃)₂O, trisilylamine, disiloxane,trisilylamine, disilane, trisilane, an alkoxysilane SiH_(x)(OR¹)_(4-x),a silanol Si(OH)_(x)(OR¹)_(4-x) (preferably Si(OH)(OR¹)₃; morepreferably Si(OH)(OtBu)₃ an aminosilane SiH_(x)(NR¹R²)_(4-x) (where x is1, 2, 3, or 4; R¹ and R² are independently H or a linear, branched orcyclic C1-C6 carbon chain; preferably TriDMAS, BTBAS, and/or BDEAS), andany combination thereof. The targeted film may alternatively containGermanium (Ge), in which case the above-mentioned Si-containing reactantspecies could be replaced by Ge-containing reactant species.

When the desired lanthanide-containing film also contains another metal,such as, for example and without limitation, Ti, Ta, Hf, Zr, Nb, Mg, Al,Sr, Y, Ba, Ca, As, Sb, Bi, Sn, Pb, or combinations thereof, the reactantspecies may include a metal source which is selected from, but notlimited to, metal alkyls such as SbR^(i′) ₃ or SnR^(i′) ₄ (wherein eachR^(i″) is independently H or a linear, branched, or cyclic C1-C6 carbonchain), metal alkoxides such as Sb(OR^(i))₃ or Sn(OR^(i))₄ (where eachR^(i) is independently H or a linear, branched, or cyclic C1-C6 carbonchain), and metal amines such as Sb(NR¹R²)(NR³R⁴)(NR⁵R⁶) orGe(NR¹R²)(NR³R⁴)(NR⁵R⁶)(NR⁷R⁸) (where each R¹, R², R³, R⁴, R⁵, R⁶, R⁷,and R⁸ is independently H, a C1-C6 carbon chain, or a trialkylsilylgroup, the carbon chain and trialkylsilyl group each being linear,branched, or cyclic), and any combination thereof.

The lanthanide-containing precursor and one or more reactant species maybe introduced into the reaction chamber simultaneously (chemical vapordeposition), sequentially (atomic layer deposition), or in othercombinations. For example, the lanthanide-containing precursor may beintroduced in one pulse and two additional metal sources may beintroduced together in a separate pulse [modified atomic layerdeposition]. Alternatively, the reaction chamber may already contain thereactant species prior to introduction of the lanthanide-containingprecursor. The reactant species may be passed through a plasma systemlocalized remotely from the reaction chamber, and decomposed toradicals. Alternatively, the lanthanide-containing precursor may beintroduced to the reaction chamber continuously while other metalsources are introduced by pulse (pulsed-chemical vapor deposition). Ineach example, a pulse may be followed by a purge or evacuation step toremove excess amounts of the component introduced. In each example, thepulse may last for a time period ranging from about 0.01 s to about 10s, alternatively from about 0.3 s to about 3 s, alternatively from about0.5 s to about 2 s.

In one non-limiting exemplary atomic layer deposition type process, thevapor phase of a lanthanide-containing precursor is introduced into thereaction chamber, where it is contacted with a suitable substrate.Excess lanthanide-containing precursor may then be removed from thereaction chamber by purging and/or evacuating the reactor. An oxygensource is introduced into the reaction chamber where it reacts with theabsorbed lanthanide precursor in a self-limiting manner. Any excessoxygen source is removed from the reaction chamber by purging and/orevacuating the reaction chamber. If the desired film is a lanthanideoxide film, this two-step process may provide the desired film thicknessor may be repeated until a film having the necessary thickness has beenobtained.

Alternatively, if the desired film is a lanthanide metal oxide film, thetwo-step process above may be followed by introduction of the vapor of ametal precursor into the reaction chamber. The metal precursor will beselected based on the nature of the lanthanide metal oxide film beingdeposited and may include a different lanthanide-containing precursor.After introduction into the reaction chamber, the metal precursor iscontacted with the substrate. Any excess metal precursor is removed fromthe reaction chamber by purging and/or evacuating the reaction chamber.Once again, an oxygen source may be introduced into the reaction chamberto react with the second metal precursor. Excess oxygen source isremoved from the reaction chamber by purging and/or evacuating thereaction chamber. If a desired film thickness has been achieved, theprocess may be terminated. However, if a thicker film is desired, theentire four-step process may be repeated. By alternating the provisionof the lanthanide-containing precursor, metal precursor, and oxygensource, a film of desired composition and thickness can be deposited.

The lanthanide-containing films or lanthanide-containing layersresulting from the processes discussed above may include Ln₂O₃,(LnLn′)O₃, Ln₂O₃-Ln′₂O₃, LnSi_(x)O_(y), LnGe_(x)O_(y), (Al, Ga, Mn)LnO₃,HfLnO_(x) or ZrLnO_(x). Preferably, the lanthanide-containing film mayinclude HfErO_(x), ZrErO_(x), HfYbO_(x), or ZrYbO_(x). One of ordinaryskill in the art will recognize that by judicial selection of theappropriate lanthanide-containing precursor and reactant species, thedesired film composition may be obtained.

EXAMPLES

The following non-limiting examples are provided to further illustrateembodiments of the invention. However, the examples are not intended tobe all inclusive and are not intended to limit the scope of theinventions described herein.

Example 1 Y(MeCp)₂(N^(iPr)-amd)

N^(iPr)-amd-Li was prepared by reacting di-isopropylcarbodiimide (4.47g, 35.36 mmol) in 30 mL of THF at −78° C. by slowly adding 22.1 mL(35.36 mmol) of MeLi ether solution (1.6 M). The solution was stirred at−78° C. for 30 minutes, then warmed to room temperature and furtherstirred at room temperature for 2 hours. The entire quantity of thefreshly prepared N^(iPr)-amd-Li solution was added to a flask containingY(MeCp)₂Cl (10.00 g, 35.38 mmol) in 50 mL of THF. The resulting mixturewas stirred overnight. The mixture was evaporated to dryness undervacuum. Pentane was added and stirred, followed by filtration through acolumn of Celite brand diatomaceous earth. The pentane solvent wasevaporated to dryness under vacuum to obtain a pale yellow waxy solid.The pale yellow waxy solid was sublimed at 115° C. at 14 mTorr toproduce 12.24 g, which correlates to an 89% yield. The pale yellow waxysolid melted at 30° C. and left a 1% residual mass during TGA analysismeasured at a temperature rising rate of 10° C./min in an atmospherewhich flows nitrogen at 180 mL/min. These results are depicted in FIG.1, which is a TGA graph demonstrating the percentage of weight loss withtemperature change.

Example 2 Y(iPrCp)₂(N^(iPr)-amd)

To a flask containing Y(MeCp)₃ (11.11 g, 27.07 mmol) in 60 mL ofpentane, was added a solution of N^(iPr)-amd-H (3.85 g, 27.07 mmol) in20 mL of pentane. The resulting mixture was stirred overnight. Solventsand volatiles were evaporated under vacuum. The resulting yellow liquidwas distilled at 20° C. at 8 mTorr. Yield is 11.4 g (87%). The yellowliquid left a 1% residual mass during TGA analysis measured at atemperature rising rate of 10° C./min in an atmosphere which flowsnitrogen at 180 mL/min. These results are depicted in FIG. 2, which is aTGA graph demonstrating the percentage of weight loss with temperaturechange.

Example 3 Er(MeCp)₂(N^(iPr)-amd)

A solution of N^(iPr)-amd-Li was prepared by reactingdi-isopropylcarbodiimide (10.65 g, 84.36 mmol) in 150 mL of THF at −78°C. by slowly adding 53 mL (84.36 mmol) of MeLi ether solution (1.6 M).The solution was stirred at −78° C. for 30 min, then warmed to roomtemperature and further stirred at room temperature for 2 hours. Theentire quantity of freshly prepared N^(iPr)-amd-Li solution was added toa flask containing Er(MeCp)₂Cl (30.45 g, 83.36 mmol) in 250 mL of THF.The resulting mixture was stirred overnight. The mixture was evaporatedto dryness under vacuum. Pentane was added and stirred, followed byfiltration through a column of Celite brand diatomaceous earth. Thepentane solvent was evaporated to dryness under vacuum to obtain a pinksolid. The pink solid was sublimed at 95-115° C. at 12 mTorr to produce34.3 g, which correlates to 87% yield. The pink solid melted at 36° C.and left a 2.5% residual mass during TGA analysis measured at atemperature rising rate of 10° C./min in an atmosphere which flowsnitrogen at 180 mL/min. These results are depicted in FIG. 3, which is aTGA graph demonstrating the percentage of weight loss with temperaturechange.

Example 4 Er(MeCp)₂(N^(iPr)-amd)

To a flask containing Er(MeCp)₃ (11.54 g, 28.12 mmol) in 60 mL ofpentane, was added a solution of N^(iPr)-amd-H (4.00 g, 128.12 mmol) in20 mL of pentane. The resulting mixture was stirred overnight. Solventsand volatiles were evaporated under vacuum. The resulting pink solid wasdistilled at 95-115° C. at 12 mTorr. Yield was 11.4 g (87%).

Example 5 Er(MeCp)₂(N^(tBu)-amd)

A solution of N^(tBu)-amd-Li was prepared by reacting1,3-di-tert-butylcarbodiimide (1.28 g, 8.31 mmol) in 30 mL of THF at−78° C. by slowly adding 5.2 mL (8.31 mmol) of MeLi ether solution (1.6M). The solution was stirred at −78° C. for 30 minutes, then warmed toroom temperature and further stirred at room temperature for 2 hours.The entire quantity of freshly prepared N^(tBu)-amd-Li solution wasadded to a flask containing Er(MeCp)₂Cl (3.00 g, 8.31 mmol) in 25 mL ofTHF. The resulting mixture was stirred overnight. The mixture wasevaporated to dryness under vacuum. Pentane was added and stirred,followed by filtration through a column of Celite brand diatomaceousearth. The pentane solvent was evaporated to dryness under vacuum toobtain an orange solid. The orange solid was sublimed at 100-150° C. at10 mTorr to produce 2.61 g, which correlates to a 64% yield. The orangesolid melted at 100° C. and left a 1.8% residual mass during TGAanalysis measured at a temperature rising rate of 10° C./min in anatmosphere which flows nitrogen at 180 mL/min. These results aredepicted in FIG. 4, which is a TGA graph demonstrating the percentage ofweight loss with temperature change.

Example 6 Er(EtCp)₂(N^(iPr)-amd)

To a flask containing Er(EtCp)₃ (20.00 g, 44.77 mmol) in 200 mL ofpentane, was added a solution of N^(iPr)-amd-H (6.37 g, 44.77 mmol) in50 mL of pentane. The resulting mixture was stirred overnight. Solventsand volatiles were evaporated under vacuum. The resulting pink liquidwas distilled at 72-74° C. at 8 mTorr. Yield is 16.4 g (67%). Themelting point was 18° C. The pink liquid left a 2% residual mass duringTGA analysis measured at a temperature rising rate of 10° C./min in anatmosphere which flows nitrogen at 180 mL/min. These results aredepicted in FIG. 5, which is a TGA graph demonstrating the percentage ofweight loss with temperature change.

Example 7 Er(MeCp)₂(N^(iPr)-fmd)

A solution of N^(iPr)-fmd-Li was prepared by reactingdi-isopropylformamidine (10.00 g, 7.80 mmol) in 40 mL of THF at −78° C.by slowly adding 4.9 mL (7.80 mmol) of MeLi ether solution (1.6 M). Thesolution was stirred at −78° C. for 30 minutes, then warmed to roomtemperature and further stirred at room temperature for 2 hours. Theentire quantity of the freshly prepared N^(iPr)-fmd-Li solution wasadded to a flask containing Er(MeCp)₂Cl (2.81 g, 7.80 mmol) in 50 mL ofTHF. The resultant mixture was stirred overnight. The mixture wasevaporated to dryness under vacuum. Pentane was added and stirred,followed by filtration through a column of Celite brand diatomaceousearth. The pentane solvent was evaporated to dryness under vacuum toobtain a pink solid. The pink solid was sublimed at 60-80° C. at 3 mTorrto obtain 2.2 g, which correlated to a 62% yield. The pink solid meltedat 50° C. and left a 5% residual mass during TGA analysis measured at atemperature rising rate of 10° C./min in an atmosphere which flowsnitrogen at 180 mL/min. These results are depicted in FIG. 6, which is aTGA graph demonstrating the percentage of weight loss with temperaturechange.

Example 8 Yb(MeCp)₂(N^(iPr)-amd)

A solution of N^(iPr)-amd-Li was prepared by reactingdi-isopropylcarbodiimide (6.88 g, 54.54 mmol) in 100 mL of THF at −78°C. by slowly adding 34.1 mL (54.54 mmol) of MeLi ether solution (1.6 M).The solution was stirred at −78° C. for 30 minutes, then warmed to roomtemperature and further stirred at room temperature for 2 hours. Theentire quantity of freshly prepared N^(iPr)-amd-Li solution was added toa flask containing Yb(MeCp)₂Cl (20.00 g, 54.54 mmol) in 120 mL of THF.The resultant mixture was stirred overnight. The mixture was evaporatedto dryness under vacuum. Pentane was added and stirred, followed byfiltration through a column of Celite brand diatomaceous earth. Thepentane solvent was evaporated to dryness under vacuum to obtain anorange solid. The orange solid was sublimed at 120° C. at 25 mTorr toproduce 22.4 g, which correlates to an 87% yield. The orange solidmelted at 36° C. and left a 3% residual mass during TGA analysismeasured at a temperature rising rate of 10° C./min in an atmospherewhich flows nitrogen at 180 mL/min. These results are depicted in FIG.7, which is a TGA graph demonstrating the percentage of weight loss withtemperature change.

Example 9 Yb(MeCp)₂(N^(tBu)-amd)

A solution of N^(tBu)-amd-Li was prepared by reacting1,3-di-tert-butylcarbodiimide (1.26 g, 8.18 mmol) in 30 mL of THF at−78° C. by slowly adding 5.1 mL (8.18 mmol) of MeLi ether solution (1.6M). The solution was stirred at −78° C. for 30 minutes, then warmed toroom temperature and further stirred at room temperature for 2 hours.The entire quantity of freshly prepared N^(tBu)-amd-Li solution wasadded to a flask containing Yb(MeCp)₂Cl (3.00 g, 8.18 mmol) in 25 mL ofTHF. The resulting mixture was stirred overnight. The mixture wasevaporated to dryness under vacuum. Pentane was added and stirred,followed by filtration through a column of Celite brand diatomaceousearth. The pentane solvent was evaporated to dryness under vacuum toobtain an orange solid. The orange solid was sublimed at 125° C. at 10mTorr to produce 1.73 g, which correlates to a 43% yield. The orangesolid melted at 103° C. and left a 1.8% residual mass during TGAanalysis measured at a temperature rising rate of 10° C./min in anatmosphere which flows nitrogen at 180 mL/min. These results aredepicted in FIG. 8, which is a TGA graph demonstrating the percentage ofweight loss with temperature change.

Example 10 Yb(EtCp)₂(N^(iPr)-amd)

To a flask containing Yb(EtCp)₃ (15.90 g, 35.15 mmol) in 250 mL ofpentane, was added a solution of N^(iPr)-amd-H (5.00 g, 35.15 mmol) in40 mL of pentane. The resulting mixture was stirred overnight. Solventsand volatiles were evaporated under vacuum. The resulting orange liquidwas distilled at 110° C. at 10 mTorr. Yield is 15.00 g (85%). Themelting point was 39° C. The orange liquid left a 3.5% residual massduring TGA analysis measured at a temperature rising rate of 10° C./minin an atmosphere which flows nitrogen at 180 mL/min. These results aredepicted in FIG. 9, which is a TGA graph demonstrating the percentage ofweight loss with temperature change.

Example 11 Yb(EtCp)₂(N^(iPr)-fmd)

To a flask containing Yb(EtCp)₃ (6.00 g, 13.26 mmol) in 20 mL oftoluene, was added slowly a solution of N^(iPr)-fmd-H (1.7 g, 13.26mmol) in 20 mL of toluene. The resulting mixture was stirred overnight.Solvents and volatiles were evaporated under vacuum. The resultingorange liquid was distilled at 120° C. at 6 mTorr. Yield is 5.9 g (97%).The orange liquid left a 1.4% residual mass during TGA analysis measuredat a temperature rising rate of 10° C./min in an atmosphere which flowsnitrogen at 180. These results are depicted in FIG. 10, which is a TGAgraph demonstrating the percentage of weight loss with temperaturechange.

Example 12 Yb(iPrCp)₂(N^(iPr)-fmd)

To a flask containing Yb(EtCp)₃ (3.00 g, 6.07 mmol) in 20 mL of toluene,was added slowly a solution of N^(iPr)-fmd-H (0.78 g, 6.07 mmol) in 20mL of toluene. The resulting mixture was stirred overnight. Solvents andvolatiles were evaporated under vacuum. The resulting orange liquid wasdistilled at 140° C. at 20 mTorr. Yield is 2.5 g (80%). The orangeliquid left a 2% residual mass during TGA analysis measured at atemperature rising rate of 10° C./min in an atmosphere which flowsnitrogen at 180 mL/min. These results are depicted in FIG. 11, which isa TGA graph demonstrating the percentage of weight loss with temperaturechange.

Example 13 Er(MeCp)₂(iPr-N—C(Me)=N-iPr)

The lanthanide-containing precursor of Example 3,Er(MeCp)₂(iPr-N—C(Me)=N-iPr), and the reactant O₃ were used to deposit afilm of Er₂O₃ on a SiO₂/Si substrate. The SiO₂/Si substrate wasmaintained at a temperature of 275° C. The pink solid precursor wasvaporized in a bubbler maintained at 115° C. The ALD cycle included aprecursor pulse of 10 seconds, followed by a 5 second purge, followed bya reactant pulse of 2 seconds, followed by a 5 second purge. The Er₂O₃growth rate was observed to be 1.2 Å/cycle. The ALD regime was assessedup to 275° C. with a deposition rate as high as 1.2 Å/cycle.

Example 14 Er(EtCp)₂(iPr-N—C(Me)=N-iPr)

The lanthanide-containing precursor of Example 6,Er(EtCp)₂(iPr-N—C(Me)=N-iPr), and the reactant O₃ were used to deposit afilm of Er₂O₃ on a SiO₂/Si substrate. The SiO₂/Si substrate wasmaintained at a temperature of 250° C. The pink liquid precursor wasvaporized in a bubbler maintained at 115° C. The ALD cycle included aprecursor pulse of 10 seconds, followed by a 5 second purge, followed bya reactant pulse of 2 seconds, followed by a 5 second purge. The Er₂O₃growth rate was observed to be 0.3 Å/cycle. The ALD regime was assessedup to 275° C. with a deposition rate as high as 0.3 Å/cycle.

Example 15 Yb(MeCp)₂(iPr-N—C(Me)=N-iPr)

The lanthanide-containing precursor of Example 8,Yb(MeCp)₂(iPr-N—C(Me)=N-iPr), and the reactant H₂O were used to deposita film of Yb₂O₃ on a SiO₂/Si substrate. The SiO₂/Si substrate wasmaintained at a temperature of 250° C. The orange solid precursor wasvaporized in a bubbler maintained at 115° C. The ALD cycle included aprecursor pulse of 3 seconds, followed by a 5 second purge, followed bya reactant pulse of 2 seconds, followed by a 10 second purge. The Yb₂O₃growth rate was observed to be 1.0 Å/cycle. The ALD regime was assessedup to 275° C. with a deposition rate as high as 1.0 Å/cycle.

Example 16 Yb(EtCp)₂(iPr-N—C(Me)=N-iPr)

The lanthanide-containing precursor of Example 10,Yb(EtCp)₂(iPr-N—C(Me)=N-iPr), and the reactant H₂O were used to deposita film of Yb₂O₃ on a SiO₂/Si substrate. The SiO₂/Si substrate wasmaintained at a temperature of 250° C. The orange liquid precursor wasvaporized in a bubbler maintained at 115° C. The ALD cycle included aprecursor pulse of 10 seconds, followed by a 5 second purge, followed bya reactant pulse of 2 seconds, followed by a 10 second purge. The Yb₂O₃growth rate was observed to be 1.0 Å/cycle. The ALD regime was assessedup to 250° C. with a deposition rate as high as 1.0 Å/cycle.

Example 17 Ce(iPrCp)₂(iPr-N—C(H)═N-iPr)

Step 1: A 500 mL Schlenk flask was charged with CeCl₃ (9.91 g, 40.21mmol) and 100 mL THF. The mixture was stirred at Room Temperature (RT)for 5 hours to form a thick oily suspension. In a separate 500 mLSchlenk flask, Na(iPrCp) (10.60 g, 81.44 mmol) was dissolved in 150 mLTHF to produce a dark pink solution. The Na(iPrCp) solution was added tothe CeCl₃.THF_(x) suspension at RT dropwise. The resulting yellow,cloudy mixture was stirred overnight at RT to give a green mixture.

Step 2: A 200 mL Shlenk flask and a 150 mL graduated dropping funnelwere assembled. iPr-N═C═N-iPr (6.3 mL, 40.69 mmol) was charged to theSchlenk flask and followed by 100 mL of THF. To the iPr-N═C═N-iPrsolution in a dry ice bath was added dropwise 27 mL of MeLi (1.6 M indiethyl ether) from the dropping funnel over 30 mins. The resultingyellowish solution was brought to RT and stirred for 3 hrs.

The flask contents of Step 2 were transferred to step 1's flask by acannula at room temperature. The mixture was stirred overnight to becomeorange solution. The solvent was evaporated by vacuum and concentratedto about 30% of the original volume. ca. 300 mL of pentane was added andthe mixture was stirred RT. The stirring was stopped and the precipitatewas settled down to give a yellowish precipitate and a deep red pentanesolution. The red solution layer was filtered by a filter cannula. Thefiltrate was concentrated to ca. 50 mL. The resulting solution wastransferred to a 50 mL flask with a 14/20 joint. A distillation head anda receiver were installed onto the flask. Solvents and volatiles wereevaporated under vacuum at 50° C. The resulting deep red oil was vacuumdistilled at 130-133° C. at 35-40 mTorr. Yield is 7.8 g (39%).

The deep red oil left a <1% residual mass during TGA analysis measuredat a temperature rising rate of 10° C./min in an atmosphere which flowsnitrogen at 180 mL/min. These results are depicted in FIG. 12, which isa TGA graph demonstrating the percentage of weight loss with temperaturechange.

Example 18 Pr(iPrCp)₂(iPr-N—C(H)═N-iPr)

To a flask containing Pr(iPrCp)₃ (5.00 g, 10.8 mmol) in 60 mL oftoluene, was added a solution of N^(iPr)-amd-H (1.54 g, 10.8 mmol) in 20mL of toluene. The resulting mixture was refluxed while stirring for 8h. Reaction mixture was cooled to room temperature. Solvents andvolatiles were evaporated under vacuum. The resulting green liquid wasdistilled at 125-140° C. at 50-55 mTorr. Yield is 3.5 g (68%).

The green liquid left a <1% residual mass during TGA analysis measuredat a temperature rising rate of 10° C./min in an atmosphere which flowsnitrogen at 180 mL/min. These results are depicted in FIG. 13, which isa TGA graph demonstrating the percentage of weight loss with temperaturechange.

While embodiments of this invention have been shown and described,modifications thereof can be made by one skilled in the art withoutdeparting from the spirit or teaching of this invention. The embodimentsdescribed herein are exemplary only and not limiting. Many variationsand modifications of the composition and method are possible and withinthe scope of the invention. Accordingly the scope of protection is notlimited to the embodiments described herein, but is only limited by theclaims which follow, the scope of which shall include all equivalents ofthe subject matter of the claims.

What is claimed is:
 1. A method of forming a lanthanide-containing filmon a substrate, the method comprising the steps of: providing a reactorhaving at least one substrate disposed therein; introducing at least onelanthanide-containing precursor, the lanthanide containing precursorhaving the formula:Ln(R¹Cp)_(m)(R²—N—C(R⁴)═N—R²)_(n) wherein Ln is Ce or Pr; R¹ and R² isindependently selected from the group consisting of H and a C1-C5 alkylchain; R⁴ is selected from the group consisting of H and Me; and n and mrange from 1 to 2; and contacting the lanthanide-containing precursorand the substrate to form a lanthanide-containing film on at least onesurface of the substrate using a deposition process.
 2. The method ofclaim 1, wherein the lanthanide-containing film is selected from thegroup consisting of Ln₂O₃, (LnLn′)O₃, Ln₂O₃-Ln′₂O₃, LnSi_(x)O_(y),LnGe_(x)O_(y), (Al, Ga, Mn)LnO₃, HfLnO_(x), and ZrLnO_(x), wherein Lnand Ln′ are different.
 3. The method of claim 1, wherein the depositionprocess is a chemical vapor deposition process.
 4. The method of claim1, wherein the deposition process is an atomic layer deposition process.5. The method of claim 1, further comprising the steps of: a) providingat least one reactant species into the reactor, wherein said reactantspecies is an oxygen containing fluid; and b) reacting saidlanthanide-containing precursor with said reactant species.
 6. Themethod of claim 1, further comprising introducing a metal precursor intothe reactor, wherein the metal precursor is different than thelanthanide-containing precursor and depositing at least part of themetal precursor to form the lanthanide-containing layer on the one ormore substrates.
 7. The method of claim 6, wherein a metal of the metalprecursor is selected from the group consisting of Hf, Si, Al, Ga, Mn,Ti, Ta, Bi, Zr, Pb, Nb, Mg, Sr, Y, Ba, Ca, a lanthanide, andcombinations thereof.
 8. The method of claim 1, wherein R² of thelanthanide-containing precursor is iPr.
 9. The method of claim 8,wherein R⁴ of the lanthanide-containing precursor is Me.
 10. The methodof claim 1, wherein m of the lanthanide-containing precursor is 2 and nis
 1. 11. The method of claim 10, wherein R¹ of thelanthanide-containing precursor is selected from the group consisting ofMe, Et, and iPr.
 12. The method of claim 10, wherein R¹ is iPr.
 13. Themethod of claim 11, wherein R² is selected from the group consisting ofiPr and tBu.
 14. The method of claim 11, wherein R² is iPr.
 15. Themethod of claim 12, wherein R² is iPr.
 16. The method of claim 10,wherein R⁴ is Me.
 17. The method of claim 11, wherein R⁴ is Me.
 18. Themethod of claim 13, wherein R⁴ is Me.
 19. The method of claim 1, whereinLn is Ce, R¹ is iPr, R² is iPr and R⁴ is Me.
 20. The method of claim 1,wherein Ln is Pr, R¹ is iPr, R² is iPr and R⁴ is Me.